Technologies like microelectronics, micromechanics, and biotechnology have created a high demand in industry for structuring and probing specimens on the nanometer scale. On such a small scale, probing or structuring (e.g., of photomasks) is often done with electron beams that are generated and focused in electron beam devices, such as electron microscopes or electron beam pattern generators. Electron beams offer superior spatial resolution compared to photon beams due to their short wave lengths at a comparable particle energy.
One example of optical devices used in such an electron beam apparatus are round lenses. Round lenses are used for focusing the corpuscular beams in conventional electron microscopes, electron and ion beam registration devices, and ion processing and implantation devices in addition to electron beam measuring devices. The geometry of these lenses, which produce rotationally symmetrical electric and/or magnetic fields, is optimized to small aberration constants. In systems where rotationally symmetric electric or magnetic fields that are not time-dependent and are free of space charge are used exclusively to produce the lens effect, the spherical aberration of the third order and the axial chromatic aberration of the first order (first degree) cannot be corrected in principle. For electrostatic fields, a zone having less than cathode potential acts like an electrostatic mirror reflecting the electron beam. Therefore, these aberrations limit the resolution and a correction thereof is only possible if one of the restrictions regarding the fields is waived (e.g., by introducing non-round symmetries).
Multipole elements (e.g., quadrupoles) are used, for example, as adjustment means or stigmators in electron microscopes. For these elements, the requirements regarding the shape of the field-generating components (pole pieces, electrodes), the accuracy of the adjustment to the optical axis, and the long- and short-term stability are in any case substantially less than for systems for correction or reduction of spherical and chromatic aberrations. However, a high degree of accuracy is also desirable for adjustment components.
For example, Wien filters (see W. Wien, Ann. Phys. 65 (1898), page 444) can be used as correctors for charged particle beam optical systems. In such a Wien filter, electrodes and magnetic poles are simultaneously utilized to create both an electric field and a magnetic field. The two fields are tuned, or adjusted, to apply equal but opposite forces to charged particles having a certain nominal energy in the incident beam on the optical axis, so that these particles of nominal energy are not deflected. To improve the imaging properties of Wien filters, quadrupole or even multipole elements of higher order are added to the Wien filter, thus rendering the Wien filter a multipole element.
The design, manufacturing, and assembly of such combined electric-magnetic multipole elements impose extremely high requirements on dimensional precision, positioning accuracy, and stability during operation of the device comprising the multipole element. The difficulties to meet these requirements for electric-magnetic multipoles are further enhanced by the fact that the electrodes, which also serve as pole pieces, have to be placed directly within the vacuum.
In the combined electric-magnetic multipole elements, which are known from Optik 60, No. 3 (1982) page 307 to 326, the parts arranged in the vacuum (i.e., the pole pieces and the electrodes), the excited individual windings fixed on the pole piece mounting, and the supply lines are cast integrally in synthetic resin (e.g., epoxy resin) in order to reduce the gas emitting surface. However, this embedding technique has the disadvantage that, in spite of maintaining its hardness for months in a vacuum, the synthetic resin as well as the varnish of the wires, emits gas. Furthermore, the resin shrinks and becomes brittle, and this deformation influences the magnetic properties of the multipole element. In addition, costly shielding of the epoxy resin is necessary in order to avoid charging by the corpuscular beam which is difficult to realize due to its complex design. Also, the integrally formed structure of this prior art multipole element is disadvantageous in terms of maintenance and readjustment or repair because the whole element has to be replaced or readjusted instead of, for example, only the pole piece and electrode.
Due to the problems described above associated with exposing the coils to the vacuum, it is desirable to place the coils for exciting the magnetic field outside the vacuum. Hence, the introduction of a vacuum-tight separation of the coils is advantageous. Also, the electrodes have to be electrically insulated from each other and from the coils when the electrodes also serve as pole pieces. Typically, the vacuum-tight seal is provided by a welding or brazing technique or by means of a gluing or molding process. Also, O-rings are used, either alone or in combination, with some of the aforementioned techniques for providing a vacuum-tight seal.
In U.S. Pat. No. 5,021,670, the coil is mounted to an elongated part of the electrode and pole piece, located outside the beam tube. A vacuum seal is formed by metal-ceramic bonds or metal caps which are soldered to the elongated parts of the electrodes and pole pieces.
In U.S. Pat. No. 6,593,578, the elongated electrodes and pole pieces are inserted into a support structure made of ceramic. The coils are mounted on the elongated electrodes and pole pieces, outside the beam tube on spools. A vacuum-tight seal between the beam tube and the ceramic support structure is provided by brazing the pole pieces to the ceramic support structure.
However, it is difficult to maintain the positioning accuracy of the individual electrodes and pole pieces when a brazing technique is adopted. In this case, the vacuum-tight seal is formed by a metal-ceramic bond created by welding or brazing. Thus, the ceramic material is locally subject to high temperatures. These high temperatures induce tension forces in the ceramic-metal joint resulting in instabilities of the joints or even deformations. As a result, the optical properties of the multipole element deteriorate due to mechanical instabilities of the metal-ceramic joints. Also, the alignment accuracy of the pole pieces and electrodes deteriorates due to such mechanical instabilities. Furthermore, brazing is difficult because of the different thermal expansion coefficients of the metal and ceramic materials involved. In the case of a brazing or soldering technique, the even distribution of the soldering flux is a complicated problem. Moreover, these problems become even more critical when they have to provide a vacuum-tight seal. Glues or resins do not provide adequate alternatives since they suffer from the disadvantages mentioned above: a high outgassing rate, degradation during vacuum operation, and shrinking.
Furthermore, all the prior art solutions outlined above have in common the problem that during operation, particularly at higher excitation of the coils, the heat created by the coils causes mechanical instabilities and deformations of the metal-ceramic joints resulting in deformations of the electromagnetic field and, thus, drifts and instabilities of the charged particle beam. However, in some applications it is especially desirable to excite a multipole element up to a point where particles entering the multipole element follow oscillating trajectories inside the multipole element. For this purpose, relatively high excitation of the multipole element is necessary.
Accordingly, there is a need to overcome the disadvantages associated with the prior art.